First, remember that communism in Eastern Europe was put in place by the occupying Soviet Army at the end of World War II, so it was never an organic or popular movement in these countries to begin with. They remained, until the late 1980s, more or less satellites of the USSR. Rebellions in Hungary, East Germany, Czechoslovakia and Poland were all crushed.
So this artificial system was widely despised, the economies of these countries largely unrecovered from the damage of World War II, and soldiers and secret police were required to maintain order. After Gorbachev loosened Soviet control over East Europe in the 1980s, the dam slowly broke open.
Elections put some reformers in government, and Hungary opened its borders with the West. This caused members of other Soviet states to flee their own countries to cross the Hungarian frontier and it was downhill from there. When it became clear the Soviets would not intervene to preserve communist power, (mostly) peaceful revolutions broke out across the entire Soviet bloc.